One known resistor of the same category is disclosed in the Japanese Laid-open Patent publication No. H4-102302.
The conventional resistor and a method of manufacturing the resistor are described in the following with reference to drawings.
FIG. 8 is a sectional view of the conventional resistor.
In FIG. 8, first upper-surface electrode layers 2 are provided on the right and the left ends of the upper surface of the insulating substrate 1; a resistor layer 3 is provided partially overlapping on the first upper-surface electrode layers 2; a first protective layer 4 is provided to cover only the whole surface of the resistance layer 3; a trimming groove 5 for correcting the resistance is provided by cutting through the resistor layer 3 and the first protective layer 4; a second protective layer 6 is provided to cover only the upper surface of the first protective layer 4; second upper-surface electrode layers 7 are provided on the upper surface of the first upper-surface electrode layers 2 so as to spread until the end in the width of the insulating substrate 1; side electrode layers 8 are provided on the side surfaces of the insulating substrate 1; nickel plated layers 9 and solder plated layers 10 are provided on the surfaces of the second upper-surface electrode layers 7 and the side electrode layers 8.
A method of manufacturing the resistor as configured above is described next, referring to drawings.
FIG. 9 illustrates process steps of manufacturing the conventional resistor.
In the first place, as shown in FIG. 9(a), first upper-surface electrode layers 2 are formed on the right and the left ends of upper surface of the insulating substrate 1, using a printing process.
Then, as shown in FIG. 9(b), a resistor layer 3 is formed by a printing process on the upper surface of the insulating substrate 1 so that part of the resistor layer overlaps on the first upper-surface electrode layers 2.
As shown in FIG. 9(c), a first protective layer 4 is formed by a printing process covering only the whole surface of the resistor layer 3, and then a trimming groove 5 is formed by cutting through the resistor layer 3 and the first protective layer 4 using a laser, or other means, in order to adjust the overall resistance of the resistance layer 3 to be falling within a certain predetermined range.
A second protective layer 6 is formed by a printing process covering only the upper surface of the first protective layer 4, as shown in FIG. 9(d).
As shown in FIG. 9(e), a second upper-surface electrode layer 7 is formed on the upper surface of the first upper-surface electrode layer 2 by a printing process so that the electrode layer stretches to the ends of the insulating substrate 1.
As shown in FIG. 9(f), a side electrode layer 8 is formed by a coating process covering the right and the left side end surfaces of the first upper-surface electrode layer 2 and the insulating substrate 1, electrically coupling with the first and the second upper-surface electrode layers 2 and 7.
Finally, surfaces of the second upper-surface electrode layer 7 and the side electrode layer 8 are plated with nickel, and then with solder, for forming a nickel plated layer 9 and a solder plated layer 10. The conventional resistors are manufactured through the above described process steps.
However, with the conventional resistors having the above described configuration and manufactured through the conventional procedure, where a trimming groove 5 has been formed by cutting the resistance layer 3 and the first protective layer 4 with a laser or other means to improve the resistance accuracy, a current noise is generated in the resistor.
Now, the mechanism of current noise generation is described in the following with reference to drawings.
FIG. 10(a) shows a relationship between the resistance correction ratio and the current noise, exhibited by a 1005 size, 10 k.OMEGA. resistor having the conventional configuration, manufactured through the conventional process. The graph indicates that the current noise characteristic gets worse along with an increasing ratio of the resistance correction. Basically, an increased ratio of the resistance correction results in a reduction in the effective resistance area of the resistor layer, which eventually leads to a ski deteriorated current noise characteristic. In reality, however, extent of the deterioration in the current noise characteristic is more than what the basic principle explains. The resistor layer is damaged by the heat generated during the resistance correction in the area around the trimming groove, and by the micro cracks caused thereby. The wide dispersion of the current noise started after the resistance correction, as shown in FIG. 10(a), represents a dispersion existing in the extent of deterioration of the resistance layer.
FIGS. 10(b), (c) show shift of the current noise generated in the resistor layer measured after the respective process steps;
FIG. 10(b) represents a resistor whose second protective layer is formed of a resin, FIG. 10(c) represents a resistor whose second protective layer is formed of a glass. The deterioration of current noise characteristic stems from the trimming process, as described earlier. In a resistor having second resin protective layer, the deteriorated current noise characteristic remains as it is until the stage of finished resistor.
Whereas, in a resistor having second glass protective layer, although a sufficient amount of heat that is required for restoring the resistance is provided at the baking process for the second protective layer the deteriorated resistor layer is hardly repaired, because the resistor layer has been covered by the first protective layer which was already baked and the glass component can not permeate into micro cracks of the resistor layer generated during the trimming operation. Namely, the current noise is hardly restored.
The current noise may be restored if the baking temperature is raised to a level at which the glass component contained in the resistor layer softens to repair the micro cracks. In this case, however, a resistance accuracy achieved by the trimming operation can not stay as it is until the stage of finished resistor.
As described in the foregoing, a problem with the conventional resistors configured above and manufactured by a conventional method to provide a certain predetermined resistance is the increased current noise due to the heat and micro cracks generated at the vicinity of the trimming groove during the resistance correcting operation.
The present invention addresses the above problem and aims to provide a resistor, as well as the method of manufacturing, that is superior in both the current noise characteristic and the resistance accuracy.